1. Field of the Invention
The present invention relates to a technique for converting a color representation used in spot color printing into a color representation which does not include the spot color. More particularly, the invention relates to a technique for appropriately determining color density values when colors are overprinted.
2. Description of the Background Art
A large number of color printed materials are created by the use of plates (or process plates) prepared for respective basic colors known as process colors (typically the following four colors: C (cyan); M (magenta); Y (yellow); and K (black)) (or by the use of data about color density values corresponding to the plates for the respective color components in plateless printing). When there is a need for printing using a color which cannot be sufficiently represented by the process plates or a color which is not permitted to vary, it is customary to prepare a special ink of a color known as a “spot color” and to do printing using an additional plate specific to spot color printing for the purpose of accurately representing the required color. In such a case, it is needless to say that an output apparatus is capable of the spot color printing.
For creation of printed materials, proof output is typically produced for the purpose of color proof and the like prior to final output. Alternatively, a simulation can be performed on the color representation for spot color printing in a layout step. For the output (referred to hereinafter as “proof output” including the latter unless otherwise specified) intended for such purposes, only a small number of copies is required to be outputted, and it is a common practice to use an output apparatus (also referred to as a “proofer” when used in the proof output) different from that for final output which is assumed to output a large number of copies. However, many output apparatuses of such a type are not capable of producing output including the spot color, but are capable of producing only output using the process plates. Thus, such output apparatuses, when used for the proof output of printed materials including the spot color, cannot produce the output for a spot color image and finally cannot produce the output which meets the purpose of checking the color representation and the like. To attain the output including the spot color in a pseudo manner even in such output apparatuses, it has been a customary practice to perform the process of converting the color density value (or the dot percentage) of the spot color into the color density values of the respective process colors before producing the output. The process of converting the color density value of the spot color into the color density values of the respective process colors in this manner is also referred to as “pseudocolorization.” Various techniques for such a color conversion process are known in the art (as disclosed, for example, in Japanese Patent Application Laid-Open No. 2001-157074, Japanese Patent Application Laid-Open No. 2003-78778, and Japanese Patent Application Laid-Open No. 10-248017 (1998)).
When color designation is determined so that there is no overlap between the ink of the spot color and the ink of other colors for the creation of a printed material by the use of the spot color, printing for the spot color plate is substantially the same as single-color printing. In this case, the use of the color density values of the respective process colors obtained by the pseudocolorization in a portion where the spot color is designated for the proof output after the pseudocolorization will achieve the output using only the process colors which represent the spot color as faithfully as possible.
On the other hand, when color designation is determined so that the spot color and other colors are overprinted, the proof output involving the pseudocolorization necessitates the representation of the overprint using only the process colors. Each of the color density values for the proof output is obtained by combining the color density value of a process color which is obtained by the pseudocolorization (and which should represent the spot color as in a pseudo manner as possible) with the color density value (for final output) originally designated for the process color by using an appropriate method. The color density values obtained by the combination are required to represent the overprint as faithfully as possible.
Japanese Patent Application Laid-Open No. 2001-157074, Japanese Patent Application Laid-Open No. 2003-78778 and Japanese Patent Application Laid-Open No. 10-248017 (1998) disclose a common technique in providing a color density value (referred to hereinafter as a “pseudocolor density value”) of each process color obtained by the pseudocolorization (or in a color space for the proofer) for a spot color for use in printing by various methods to combine the pseudocolor density value with a color density value (referred to hereinafter as an “original color density value”) for final output which is provided for each process color by a predetermined method, thereby providing a color density value (referred to hereinafter as a “proof color density value”) of each process color for use in proof output.
A relatively simple form of such combination is a method (refereed to hereinafter as an “Add combination method”) of simply determining the sum of the pseudocolor density value and the original color density value as the proof color density value for each color component. Japanese Patent Application Laid-Open No. 2001-157074 illustrates the application of the Add combination method in an RGB color space and L*a*b* space. When this process is performed in a CMYK color space generally used for printing, there can be cases where the sum calculated percentagewise exceeds 100%. However, the color density is saturated at 100% in the actual printing. Therefore, this method is not capable of representation of gradation which reflects the overprint.
Another conceivable method that is similarly simple is a method (referred to hereinafter as a “MAX combination method”) of making a comparison between the pseudocolor density value and the original color density value per pixel for each process color component to adopt the greater value as the proof color density value. Japanese Patent Application Laid-Open No. 2003-78778 discloses the process of generating pieces of halftone data about a process color and a spot color and then setting post-combination halftone data at “1” for a region in which one of the pieces of halftone data is “1.” This process is substantially equivalent to the MAX combination method. In the MAX combination method, the color density values do not exceed 100%. Additionally, a proof color density value similar to that obtained by the Add combination method is obtained for a region in which at least one of the color density values equals zero, but the smaller one of the color density values is different from the original output for a region in which neither of the color density values equals zero. This presents the problem that the overprint cannot be said to be faithfully represented.
Japanese Patent Application Laid-Open No. 10-248017 (1998) illustrates a technique such that, when neither the pseudocolor density value nor the original color density value equals zero, the order of overprinting is considered and the color density value for an image forming the top layer is adopted (although the color density values are represented as vectors). This technique, however, is similar to the MAX combination method in that the overprint cannot be said to be faithfully represented because the remaining density value is neglected.